I bumped into a video on YouTube the other day covering a topic I have been contemplating. The topic? Joining Beams. Or more specifically, the topic of research is "How to Join Beams at the ends", like to extend a beam. Or, maybe better put, what is the cheapest, easiest, most robust way to join beams?
The context of this research is looking at 2 areas of engineering with trailers: 1) The attachment of a gooseneck to the main beams of a trailer; 2) Attachment of a dovetail at the end of a trailer.
I have my methods, and I have heard others discuss theirs; so, I decided to do some research to see what others think. We might even get lucky and find some technical stuff about how to join beams.
In Trailer Engineering – well, at least here at Syntheses – we focus on ways to do things better. Learning is awesome. It is easy enough to see what others have done, then engage in R&D (Ripoff & Duplicate). Ah, but there is always a better way to do things, so we like to look for good ideas everywhere. Not that we believe we can devise the ultimate solution, but we continually seek opportunities for learning and improvement.
The Video That Caught My Attention
The video is titled "The Correct Way to Make Welded Splice Joints in Chassis Members." It is not exactly the conditions I am thinking about to join beams, but it is close. What can we learn?
While the video is perhaps a little laborious for someone who is not familiar with the math, you do not have to be an engineer to understand it. I think the author did a great job in touching the subjects, while not getting bogged down in the engineering details. Geek out just a little on the math, then explain it. He delivers the concepts well.
Here is the video. A big Thank You to the author for the work that went into it.
After watching the video a couple of times, I think he has done a commendable job with the engineering, and I extend a heartfelt THANK YOU for the effort.
Throughout the video, he poses questions about the optimal shape for splicing (joining) a beam. Overall I agree with his assessment from a purely theoretical standpoint - there should be no reason to make a diagonal, or to use steps.
I think his look at wood beams for comparison is rather insightful. I had not considered that years of wooden beams would influence thoughts for metal beams still today. Especially since wood beams have a very different technique of connecting - not welding like for steel beams. And, no one alive today was around before steel beams were common. However, the two materials and their respective approaches are not that dissimilar. (We will talk about that more below.)
Answering How To Join Beams
The one thing I felt the video was lacking is a real answer to the original question. He did answer it from a theoretical standpoint, but as we know theory is not always practical.
In a practical perspective, circumstances will often change the theoretical answer. A most important case-in-point: welding alters material properties.
The analysis in the video assumes a homogeneous material, yet in reality, the material changes near the weld. Both in geometry and in material property. Welds do not penetrate perfectly through the material in a homogeneous manner. There is always a dip near the edge of a weld, changing the geometry. Also, the material heat treatment changes around the weld, and that will affect how to join beams.
Yes, welds can penetrate and do a great job of going through the material, but by the nature of the process it is not homogeneous. The same is true for heat treatment near the weld. It is not homogeneous. Instead, the material has gradients of strength and brittleness throughout the areas near a weld.
What does that mean? It means to get a true answer, our theory would have to analyze through and including the post weld material gradients. Yes, both in material state gradients from heat treatment, as well as in material fill and geometry. This is especially true when the welding rod material differs from the parent material. (Usually it does.)
Can we even know that when we join beams? That analysis is exceptionally difficult to do, so we normally handle it with proper processes and safety factors.
His video definitely takes into account and calculates the Safety Factor of the areas he is looking at. That is cool, but it does not tell us how much of the safety Factor we take up when we make a weld in an otherwise homogeneous beam.
Beam Use
A big consideration as we join beams is the intended use. A static load beam that does not see much variation is pretty rare - especially with vehicles. So how much vibration do we expect? How much oscillation is there with the load? What is the amplitude?
The video mentions the beam in a vehicle, which will usually have a complex loading over time. The same is true for a trailer frame. In these situations the potential effect of weld distortions are greater - because welds and fatigue don't play so nice with each other.
True, this is highly material dependent. Aluminum is much more sensitive to fatigue than steel, for instance. However, no matter what material we choose, failures near welds are far more frequent than failures free of any weld interaction. I have made a bunch of money in my career analyzing these kinds of failures.
Without going into any detail, we will just say that the How of the beam use is a big part in How we should join beams.
Factory Processes / DIY Processes
Since the video does touch on it a little, we can point out that in a factory you have a lot more control to join beams. Here are a few points to consider.
- First, In a factory they know what the material is, exactly, so they can choose a proper weld fill material. In DIY and aftermarket, the exact material of the original beams is often unknown. That means we join beams with whatever wire is in the MIG, or whatever rod we have laying around.
- Second, In a factory, after welding you can heat treat the entire beam, like in an oven, to normalize the material and make the properties closer to homogeneous. When a beam is cut, then spliced with something (like to extend a beam), it will never be truly homogeneous, but heat treatment can drastically minimize the effects of a weld. We see this a lot in things like bicycle frames where the parent material is quite thin. If bicycle frames are not heat treated after welding, they will fracture quite easily. From a practical standpoint most people in DIY do not have access to that kind of equipment.
- Third, when in an environment of fatigue, crack propagation often starts, and propagates right in the areas of heat distress. I have seen many beam failures occur near welds, because the material properties have changed near the weld, and the crack will propagate through and near the weld much more easily. Some say it is from the brittle areas internal where they join beams. Others say it is due to discontinuity in the material. I don't know the molecular reasons, but I definitely know the effect.
For DIY (and professionals not in a factory), I agree completely with the practicality of those who do join beams - namely the independent trailer fabricators that must do the work, AND live with the results. Skill and care matter.
In college I worked as a mechanic. The boss told me "As a mechanic, you have to be cleaner than a doctor." I pondered that for a minute, then because I looked puzzled, he finished. "Doctors get to bury their mistakes, you have to eat yours." That always stuck with me, and it is certainly true of trailer builders that must live with / or eat their choices about how they join beams.
To combat these items above, especially crack propagation, having stresses spread farther in the beam connection can reduce the effect. Here are some examples.
Geometry To Extend A Beam
For instance, an angle cut through the beam gives more area of weld. Yes, that means more area affected, but it can also mean spreading it out. Unlike the conclusion in the video, I will argue that in practice, the Geometry we choose matters.
Another aspect which seems important for failures: Notice the direction of the forces. For a beam in bending, the forces are along the beam, parallel, if you will, to the length of the beam. When you extend a beam, or join a beam, we generally like to avoid welds that are perpendicular to the direction of the forces. The discussion about halfway through this article from Mechanical Elements about welding things to trailer beams covers this concept in detail.
In the simple case, like in the video introduction image, cutting straight across the top of the beam. Even though the angle is through, the side of the beam has less of an effect in spreading the stress load. (Cut 2 below.) If the angle cut is the opposite direction (Cut 3 below), or even better as a compound angle (Cut 4 below), the spread of the weld effect is greater.
In the image, from left to right: 1. Straight cut; 2. Angled cut (looking at a side view); 3. Angle cut (looking from a top view); 4. Compound Angle (angle cut from both top and side views).
- A Straight Cut minimizes the cut perimeter, and therefore minimizes the weld length. At the point to join beams, all the weld is in the same plane of beam bending action.
- Side View Angle Cut has the same effect as the Straight Cut for the top and bottom surfaces, which are the most stressed, so would be only minimally stronger than the straight cut.
- Top View Angle Cut has more surface area for weld along the top and bottom surfaces, therefore it spreads the stress at the weld more. Yes, there is more heat affected area, so it is a bit of a toss up as to how much the strength will increase over the straight cut. However, to extend a beam, this is stronger. (Assuming good welds.)
- Compound Angle Cut (double diagonal if you prefer to think of it that way) is potentially stronger than the others simply because it spreads the weld around a longer path, and none of the path is perpendicular to the force directions.
The extent of the strength increase between these is arguable, because none of the welds above will approach the strength of a continuous beam.
I believe the video gives us a good argument about why it does not matter on how we join beams, but I believe the missing factor is the practical. From experience, I know that the angled welding is stronger than anything straight (across the high stressed surfaces when the beam is in bending). I also know that the way to success to join beams is additional material at those surfaces, minimizing weld on the high stress surfaces.
Theoretical Engineering / Practical Engineering
There are 2 really important engineering approaches. First, the theoretical engineering. (That is all the math and theory.) Second, practical engineering. These are both important, but they are not independent.
Theoretical engineering without the practical only works in school. Products do not make it in the market if the design is strictly theoretical.
Practical engineering is a little harder to describe, so we wrote a full article, not only about the practical side of engineering, but about using experience to apply it. Please read that, because it also has examples in trailer engineering and bicycle design.
Looking around, it is pretty easy to see stuff that people say is "Over Engineered", and I would say some ways trailer designers who join beams often fit that paradigm. Truly, we can go overboard on the practical by ignoring the theoretical, but that misses the optimization we hope to find as we look for the best ways to extend a beam.
Think about it. Fractures are not usually in the weld, instead, near the weld where the heat affected metal is not supported with the added mass of the weld. To know this, and to design for it is the practical engineering.
Practical Compensation for a Theoretical Problem
Part of the answer for the question of the video, is definitely in the changes to material properties caused by welding. Basically, as we join beams by welding we are placing an interruption in the "flow" of stresses along the beam.
This is not unlike the wood beams of old. To join beams of wood, it is most practical to create some sort of overlapping joint - pin joint, finger joint, dovetail, etc. These increase the surface area to join, and at the same time create a path that is much more difficult for a crack to follow.
They do this by changing the geometry. We can do the same with weld connections as we join beams. The most common method with metals is to add stress plates over the joint. Usually we join the beams with weld, then use plates over the weld seam to extend a beam. Prep the beam ends with bevels to allow weld fill, then weld, then grind the welds flat before applying the plates.
Stress plates like in this image are common, and they do a great job of creating a complex joint that is hard for a crack to follow. They certainly strengthen the areas of the weld in the main beam, but they also introduce more stress with more welding.
One thing that helps is the corners turned diagonal, and it helps even more by extending them. (To a point.) See the diamond shape side plates in this image.
One important thing to consider is where the stress is greatest. This is the farthest faces from the neutral plane when in bending, which is most of the beams we are discussing. If that is where the stress is greatest, then the priority should be to mitigate interruptions there.
Stress plates like in the image address that ONLY if we avoid welding on the top and bottom surfaces. So, in the image you will note the Blue representation of welds. (Colors only to illustrate the various areas.) They are only on the sides of the plate, not on the top surface of the beam or the plate. Do not weld across the top face of the beam.
We can do something similar on the sides, but this is less important if the beam is primarily in bending vertically. Like a trailer frame main beam.
We must be super careful of adding extra stress with more welding. We can actually weaken the beam instead of strengthening it, if we put welds in the wrong places.
PLEASE NOTE: This is but one simple example to join beams. Your situation, with your forces and your beam parameters will probably be different, so treat each beam in a manner that addresses the specific needs.
Summary
Whether you need to extend a beam like for a trailer, or if you need to join beams for some other project, these principles apply. The greatest success will come as we thoughtfully consider the beam application, how the forces will apply, and the effects with our material choice. We can extend a beam successfully by applying both the theoretical and the practical design.
So, to answer the question of the video, yes, as we join beams - shape matters! Yet even more important is the treatment around the seam to mitigate the effects of our joining method.
The beginning of the video shows several examples of joining beams. I smile every time I see it. In these cases, things like bridges are exceptionally overbuilt (large safety margins) because the result of failure is the unthinkable. A vehicle chassis or trailer frame less so. In the old days, to extend a wood beam they would often wrap the joint with leather or metal to strengthen it. With welding we can do something similar - though soaking leather is not needed.
To join beams with Bolts - like some of those in the beginning of the video - is a whole different animal. Maybe we will discuss those in some future article? Who knows.
In the meantime, if you are looking for more examples or more specifics about how to join beams for a dovetail or gooseneck trailer, see the companion article on our DIY site, Mechanical Elements.
Have a wonderful day!